1. Field of the Invention
The present invention relates to pulse width modulation integrated circuits, and more particularly to a soft start pulse width modulation integrated circuit having a reduced number of external connection terminals whereby the circuit processes a soft start signal and an external sync signal through a single external connection terminal.
2. Description of the Prior Art
A switch-mode power supply (SMPS) is widely used as a power supply for many electronic appliances. A conventional SMPS employs a pulse width modulation integrated circuit and includes a soft start feature for mitigating the transient phenomena that typically occurs when the SMPS is initially operated. The soft start feature slowly increases the current or pulse width supplied by the SMPS according to specified criteria. In a typical pulse width modulation integrated circuit, the soft start function is implemented by employing a capacitor having a large capacitance. The capacitor is coupled to a designated terminal of the SMPS which then acts to limit the rising slope of the current or pulse width supplied to an electronic device. Cathode ray tube (CRT) video appliances such as television sets, monitors, etc. include vertical and horizontal deflection circuits in order to display a video signal on the CRT screen. The horizontal deflection circuit operates with a horizontal scanning frequency of several tens of KHz which is apt to interfere with the switching frequency of the SMPS. This interference causes visual noise to be produced on the screen of the CRT, thereby deteriorating the picture quality.
Generally, the switching operation of the SMPS is synchronized to the horizontal frequency in order to prevent the above described interference phenomena. In a conventional pulse width modulation integrated circuit, the soft start function and the synchronization function are performed with respective signal processes through two separate external connection terminals.
Modern miniaturization techniques have allowed the SMPS and the pulse width modulation circuit to be placed onto a single-packaged integrated circuit. Since the SMPS typically includes a MOSFET power-switching element, however, a heat sink must be integrated with the circuit to dissipate the heat generated from the switching element. This creates severe limitations when accommodating the package with a number of external connection terminals.
Accordingly, the need exists for a soft start and pulse width modulation circuit which operates using as few external connections as possible.